1. Field of the Invention
The invention generally relates to optical systems for lasers. More particularly the invention relates to mirrors exhibiting a high destruction threshold for reflection of high-power laser pulses.
2. Description of Related Art
For high-power laser systems, particularly in research, material processing, or for applications in nuclear fusion, the trend towards increasing power is unbroken. Mention may be made, e.g., of projects ELI, Apollon, and Petawatt.
In order to achieve higher and higher powers up to the petawatt range, pulse energies are increased and pulse durations are shortened. Nowadays, typical pulse durations are in the order of 10 to 200 femtoseconds.
Pulses having such pulse durations cannot be directly amplified, they have to be extended to nanosecond pulse durations, e.g. using grids, amplified in this form in active laser media, and then re-compressed.
Furthermore, these pulses do not have a single wavelength, i.e. they are not monochromatic as it is known from conventional lasers, e.g. continuous-wave lasers. Rather, the pulses comprise an entire wavelength range. This results from Fourier transform between frequencies (wavelengths) and pulse duration, or from Heisenberg's uncertainty principle. Therefore, in order to be able to re-compress a nanosecond pulse after amplification, or to cause interaction of light and matter, all wavelengths of the pulse must be present simultaneously. If the shortest or longest wavelengths components are lacking, the compressed pulse will no longer have a duration of a few femtoseconds, but will be considerably longer. However, media exhibit dispersion, that means the speeds of light of different wavelengths will generally be different. Therefore, the spectrum diverges in time and space and it becomes impossible to re-compress the different spectral components into a femtosecond pulse.
In order to preserve femtosecond pulses as such (so that they do not diverge) to be able to compress longer pulses into femtosecond pulses, or to be able to have femtosecond pulses interact with matter at a particular location, the optical components transmitting these laser pulses should exhibit a lowest possible dispersion over a sufficiently wide range of wavelengths, i.e. the components of the light pulse have to be transmitted at equal speed. For example, the short wavelengths components should not run more slowly through a dispersive medium than the long wavelengths components.
The invention is now based on the object to provide a mirror which on the one hand meets the best possible the above-mentioned requirement of a low group delay dispersion (GDD; simply referred to as dispersion below) over the wide range of wavelengths, and which on the other hand is suitable to reflect pulses of high and highest intensity.